Adenovirus-based gene transfer vectors have been increasingly developed as vaccine platforms against both old and newly emerging infections.1-3 However, the real world application of adenoviral vectors, in particular in the developing countries, is limited by their instability when stored or transported at even mild temperatures. Alteration of genetic data within viral genomes for vaccine vector applications results in an increased instability in maintaining infectious function.4,5 Storage of these vectors within synthetic vials furthermore accelerates denaturing of proteins and loss of viral infectivity through aggregation. Thus, to maintain function, adenoviral vectors suspended in an aqueous medium require storage at temperatures close to −80° C. to maintain ‘cold chain’ protocols.6 This condition inhibits molecular movements of the stored adenoviruses, hindering their aggregation.7-9 Immobilization of viral vectors within cold storage conditions is uneconomical, and potentially infeasible in areas around the globe requiring vaccination the most.
A major goal for both the World Health Organization and Bill & Melinda Gates Foundation is to alleviate cold chain requirements for vaccine storage and distribution.10 Hence, thermal stability, as used in reference to new classes of vaccines, refers to the ability of a viral vector to be stored at elevated temperatures (above −80° C.) for prolonged duration without significant loss of activity. A promising approach capable of increasing thermal stability of labile vectors is through their dispersion within the amorphous phase of a solid matrix, termed as vitrification.8,11 Vitrification of viral vectors within sugars, polymers, amino acids, surfactants, and other materials has maintained viral activity at storage temperatures above typical cold chain temperatures.12-14 
Previous studies have shown the relationship between matrix physical and chemical properties on thermal stability of entrapped species.15 The production of a solid matrix is known to greatly hinder the molecular movements of an entrapped adenoviral vector, thus preventing unfolding and aggregation16. Selection of a purely amorphous matrix may result in a solid with high moisture sensitivity17 which will reduce stabilization of any dispersed labile biological materials.18 Conversely, crystalline structures are moisture-resistant but not optimal for stabilizing dispersed biological materials due to poor incorporation within the matrix. Binary excipient mixtures are a novel consideration for stabilizing viral vectors since they can be used to balance the amorphous and crystalline phases of a formulation,19,20 though no current examples are systematically evaluated within the literature. Semicrystalline materials may offer increased thermal stability and moisture resistivity over their amorphous counterparts. Previous publications have demonstrated that crystalline regions can act as physical barriers for molecular movements and water sorption.21-23 
Several drying processes such as spray drying, freeze drying and foam drying have been employed in recent years for producing dry powder forms of solid viral vector dispersions.24-26 Spray drying is increasingly preferred since its simple requirements facilitate product scalability27 and favorable economics. During spray drying, a pressurized gas is used to disperse a liquid feed into small droplets within a drying chamber. Evaporation of heated aqueous droplets results in precipitation of the dissolved solutes and suspended materials. Current research aimed at improving thermal stability for labile biological materials has shown great success with spray drying vaccines ranging from attenuated pathogens to antigen-based formulations.24-26,28,29 The degree of thermal stabilization varies significantly depending on the dispersed biological material. For example, a spray dried bacillus Calmette-Guérin vaccine formulation with L-leucine demonstrated a minimal activity loss of approximately 2.0 log after 120 days at 25° C. under high moisture protection.24 Alternatively, an antigen-based influenza subunit vaccine stabilized in inulin retained considerable immunogenicity for up to three years of storage at 20° C.29 The variance in stability among spray dried biological materials emphasizes the need for specific evaluation of each vaccine backbone and excipient combination.
Human adenovirus type 5 (AdHu5) has been shown to be an effective vaccine vector for prevention of infectious diseases and has been developed in both liquid buffer and lyophilized forms.30,31 Current limitations to AdHu5 use stem from pre-existing AdHu5 immunity and the lack of a thermally stabilized form. It is estimated that 30-100% of the population, depending on geographical location, have been exposed to AdHu5 and therefore elicit an AdHu5-specific response upon infection.32 The anti-AdHu5 immunity pre-existing in most of the human population poses a potential limitation to the application of AdHu5-vectored vaccines. However, the results from a recent clinical vaccine trial suggest that the potency of AdHu5 vector system is able to diminish the negative effect of a pre-existing immunity.30 Furthermore, AdHu5 vector is particularly amenable to vaccination via the respiratory mucosal route against lung infectious diseases and the human respiratory tract has been found to have minimal pre-existing anti-AdHu5 immunity.33 Thus, an AdHu5-based vaccine is expected to be even more effective when given via the respiratory mucosal route versus a parenteral route. In terms of thermal stability, AdHu5 has yet to be developed into a well-stabilized spray dried form.